The purpose of oil-well safety valves is to automatically stop production of well fluid in the event of accident at the wellhead or downstream of it. Such valves are hydraulically controlled from the surface to open, and close automatically by a strong return spring the moment there is a drop, accidential or deliberate, of hydraulic pressure. These valves have been studied extensively for development and improvement, and they are widely used especially in offshore wells.
When being installed, these valves are anchored in a receiving sleeve located inside the production tube. This anchoring is usually by means of anchoring dogs of an elastic nature which can expand radially to enter an anchoring groove of the receiving sleeve. Thereupon these elastic dogs are locked in their anchoring positions by a spring which forces back a latch inside these dogs.
However, the known valves of this type suffer from drawbacks. In the first place, the anchoring procedure is laborious and carried out blindly inside the well without being assured of success at all, as the operator has no means of knowing whether the valve is properly anchored and locked in the receiving sleeve. In case of poor anchoring, the valve cannot withstand the effluent pressure in its closed position, which entails a very serious danger of the well blowing out. Again the known valves provide no assurance that they will open completely after the hydraulic control fluid has been pressurized.
In these known valves, furthermore, the elastic dogs and the latching spring of the anchoring system are immersed in the effluent, i.e. the well fluid, and therefore subjected to its deposition and its corrosiveness.
The primary object of the present invention is to create an improved safety valve free from the above cited drawbacks.
A particular object of the invention is to assure proper valve anchoring when it is being placed in the well.
Another object of the invention is to assure satisfactory valve operation with respect to opening and to permit anchoring only in case of full opening.
Another object of the invention is to permit easy valve retrieval in all cases either normal retrieval for maintenance or emergency retrieval in case of malfuntion.
Another object of the invention is to provide a valve of which the anchoring system is free of any elastic part exposed to deposition or corrosion.
To facilitate description, the valve of the invention is assumed in place in an oil well, and the expressions "high", "low", "upper", "lower" will refer to this configuration.